After becoming a full-fledged Companion, Aela gives you is this quest. Because of the Radiant Storytelling engine of Skyrim, your objective can change across multiple playthroughs. If you already have discovered the location of the objective quickly fast travel there and get rid of the animal. If not, you can still fast travel to the closest location and move towards the objective.

When you retuen to Aela and turn in the quest, it begins the next Companion quest: The Silver Hand.